1. Field of the Invention
The invention relates to predispersed fiber compositions and to the process for preparing such compositions. More particularly, the invention relates to such predispersed fiber compositions suitable for use in reinforcing rubber or plastic compounds. Such predispersed fibers disperse rapidly and uniformly into the rubber or plastic compound stock to which they are added.
2. Description of the Prior Art
Fibrous fillers are presently in use as reinforcing agents for plastic and rubber compounds. For example, asbestos, cellulose, glass, inorganic and synthetic fibers having various dimensions and pretreatments are currently employed to reinforce plastics. In the rubber industry, including elastomers generally, fibrous fillers such as cellulose, glass, inorganic and synthetic cord and fabric are commonly used to increase modulus and tensile strength in tires, belts and hose. Short fibers, such as chopped rayon, polyester and cotton are also frequently used to increase modulus in such items as V-belts.
Prior to the present invention, problems were encountered in obtaining uniform coverage and dispersion of the fibers throughout the rubber matrix during a reasonable and practical mixing cycle. The poor processibility of these fibers with rubber formulations has been a serious handicap to their acceptance as suitable reinforcing agents. Pretreated fibers are commercially available which have been coated with diisocyanates and resorcinol-formaldehyde-latex dips. Such pretreated fibers are expensive and unsatisfactory because the coating operation is performed on continuous filaments which are subsequently chopped, thus exposing untreated ends. Examples of such methods can be found in U.S. Pat. Nos. 3,639,311, 3,746,669, 3,793,130 and 3,940,357.
In U.S. Pat. No. 3,793,287, a process is disclosed for preparing paste extrudable powder mixtures of polytetrafluoroethylene and an inorganic filler. When fibers are employed as the filler, dilute aqueous dispersions of polytetrafluoroethylene are mixed with relatively minor amounts by weight of glass fibers and a specifically defined cationic substance which imparts hydrophobic properties to the filler. A mixture of polytetrafluoroethylene and filler was then precipitated from the dilute slurry by thorough stirring.
In the paper industry, natural and synthetic rubber latices have been added to slurries of fibers, such as cellulose, asbestos, cork, glass, regenerated leather and synthetic fibers, prior to sheet formation to produce products having certain desirable properties. This process is called wet-end addition. The wet-end addition process comprises preparing the pulp stock with fresh water and adjusting the pH of the stock to a minimum of 8.0 with soda ash or dilute sodium hydroxide. Various additives such as stabilizers or antioxidants may be included in the stock if desired. A latex, such as neoprene, is then thoroughly mixed into the stock and coagulated by the addition of a dilute coagulant, such as alum, reducing the pH to 4.5. Wet-end addition is usually performed at a total solids content of about 2-4 percent by weight. Widespread use of the wet-end addition process for incorporating additives and thermoplastic materials into paper has been discouraged by disadvantages such as fouling of equipment, wire and felt plugging and problems or retention causing economic losses and pollution.